fenv.h appears to be implemented as a simple library, but it actually requires major compiler support. In order to make it work, the C standard added #pragma ST

fenv.h appears to be implemented as a simple library, but it actually requires major compiler support. In order to make it work, the C standard added #pragma STDC FENV_ACCESS to the language itself, which is described in the link you posted. Compilers such as clang and GCC haven't implemented that pragma or the features it entails yet, so fenv.h doesn't work reliably in practice. The underlying problem is that programming languages and compilers want to model something like "add" as an operation which has two inputs, one output, and no side effects. The need to support flags conflicts with this. Declaring that flags don't cross function boundaries or any other boundaries doesn't make the problem go away.